Polyesters, although one of the older commercial synthetic polymer systems, continue to meet a wide range of human needs and remain industrially important. This invention relates to the preparation of synthetic organic polymers. More particularly, this invention relates to the preparation of condensation polymers such as from hydroxycarboxylic acids or from polyfunctional alcohols and polyfunctional carboxylic acids.
Condensation polymers derived from di- and other polyfunctional alcohols include, among others, saturated and unsaturated polyesters, polyester-amides, polyurethanes and polyacetals. Polyesters are a particularly preferred class of condensation polymers because their properties make them suitable for a variety of end use applications including textile fibers, films, coatings and engineering plastics. By judiciously selecting monomers and polymerization conditions, it is possible to optimize the properties desired for a particular end use.
A conventional method for preparing polyesters and other condensation polymers derived from hydroxyl containing monomers is by direct esterification, whereby the appropriate monomers, usually polyhydric alcohols or phenols and aliphatic, cycloaliphatic, or aromatic polycarboxylic acids are reacted together by melt-polycondensation at temperatures often exceeding 250.degree. C. to favorably drive the equilibria by removing the water that is formed as a by-product of the esterification reaction. The conditions required to obtain condensation polymers of the desired molecular weight are disclosed in numerous patents and other texts.
The standard industrial procedure for acquiring polyesters is to maintain the reagents in a molten state throughout the polymerization reaction. In this method, the water produced as a by-product of the reaction is continuously removed during the polymerization, generally by the use of vacuum or by azeotropic distillation.
It is known that the reaction between an alcohol and a carboxylic acid involves an equilibrium that can be represented by the equation: EQU ROH+R'COOH.revreaction.R'COOR+HOH
In the foregoing equation when R and R' are hydrocarbyl groups, the reaction is referred to as a "direct esterification".
The prior art teaches that removal of the by-product water is essential to avoid hydrolysis of the desired ester. This requirement also applies to polyesterification reactions, the only difference being that the carboxylic acid and the alcohols represented by ROH in the foregoing equation are polyfunctional.
It is also well known that the molecular weight of a polyester formed by direct esterification is determined to a large extent by the efficiency with which the by-product is removed from the reaction mixture. If the desired molecular weight is relatively low, the water can be evaporated or distilled under atmospheric pressure from a reaction mixture wherein the reagents are in a molten form and at a temperature of from about 150.degree. to 200.degree. C. or greater. This process can often be facilitated if an inert gas is passed through the reactor. To achieve the higher molecular weights desired for textile fiber production or coatings, it is usually necessary to either completely remove the water under reduced pressure or employ an organic solvent that forms an azeotropic mixture with the by-products. One disadvantage inherent in the foregoing prior art teachings is that the energy input required to remove the by-product substantially increases the cost of manufacturing polyesters. Another major disadvantage that needs mentioning is that of side reactions at the high temperatures, such as group scrambling which ruins the designed structure of the desired polymers and, double-bond coupling, etherification of carbinols, decarboxylation, ester bond pyrolysis and intermolecular scrambling through transesterification, that leads to gellation, darkening of the resin, and the like.
In addition, another disadvantage associated with preparing condensation polymers by conventional polymerization techniques is that the high viscosity exhibited by these products makes them difficult to transfer and handle.
Thus, while the melt process is efficient and economic, utility is limited to those systems capable of withstanding the high process temperatures. Polyesters with sensitive structures are obtained at lower temperatures by methods requiring special reagents that produce unwanted by-products. Common examples include polycondensation of dicarboxylic acid chlorides with diols using acid acceptors or displacement reactions of reactive halides with carboxylate salts. Until recently reference has been nonexistent to direct polyesterfication under mild conditions of carboxylic acids with carbinols without the use of special reagents. The search therefore continues for effective low temperature alternatives where the inherent advantages of direct esterification are retained, that is, where the monomers are readily available and inexpensive and where problems disposing of the by-product, water, are minimal.
The instant invention method shows a method of direct polyesterification in heterogeneous media where the by-produced water forms in a hydrophobic phase and transfers to a hydrophilic phase. The free energy released in the transfer becomes a principal driving force for the overall process. Low temperatures suppress side reactions and allow the inclusion of temperature sensitive structures that would be normally destroyed in a typical high temperature process. By judiciously selecting monomers, temperature, catalyst and amounts of water present it is possible to optimize polymer properties for a particular end use.
One method for avoiding the problems associated with the manufacture and processing of relatively high molecular weight polymers in molten or solubilized form is to employ a technique known as emulsion polymerization whereby one or more monomers are reacted in an aqueous medium containing large amounts of water, a catalyst and, usually, one or more surfactants. The final polymer is obtained as an aqueous emulsion or latex exhibiting a relatively low viscosity sometimes approaching that of water. Heretofore emulsion polymerization of organic monomers employing water as the continuous phase has been employed substantially exclusively for the polymerization of ethylenically unsaturated compounds in the presence of free radical sources, such as organic peroxides. Since the presence of even small amounts of water during condensation polymerizations involving polyhydric alcohols and polyfunctional carboxylic acids have been shown to substantially reduce the molecular weight of the resultant polyester, emulsion polymerization in aqueous media would not have been considered a practical means for preparing condensation polymers in general, and particularly polyesters and other polymers derived from polyfunctional alcohols.
Since emulsion polymerization employs an aqueous phase, this relatively large amount of water would be expected to displace the equilibrium of the polyesterification reaction in the direction of degradation of any polymer formed to the corresponding polyfunctional carboxylic acid and alcohol. It did not therefore appear obvious to employ either of these techniques as a means for preparing commercially useful polyesters. However, Saam et al in U.S. Pat. No. 4,355,154, issued on Oct. 19, 1982 and U.S. Pat. No. 4,374,953, issued Feb. 22, 1983 deal with such emulsion preparations of polyesters and polyacetals.
These references however do not teach that specifying the amount of water, or its mode of addition, might be advantageous to obtaining high conversions of carboxylic acid groups. Likewise, they do not specify that use of the catalysts that form a separate phase in the oil phase might be advantageous in obtaining high conversions either.
The '154 patent deals with a method for preparing condensation polymers by emulsion polymerization by reacting aqueous compositions containing an emulsified hydroxyacid or an emulsified mixture comprising a polyfunctional carboxylic acid or anhydride and a polyfunctional alcohol. The emulsion also contains a suitable polycondensation catalyst. The polyesters prepared by that method are alleged to be useful as plasticizers and as precursors for alkyd resins and other polymers.
The '953 patent deals with a method for preparing polyacetals and polyketals by emulsion polymerization by reacting emulsified carbonyl compounds with emulsified polyfunctional alcohols in aqueous media and in the presence of specified polycondensation catalysts. It is alleged that coherent films of the resultant polymers can applied to substrates from aqueous or non-aqueous media.
More specifically, U.S. Pat. No. 4,355,154 teaches that the amount of water is not critical (cf. column 7, lines 13 to 26) as long as the polyol is not appreciably soluble. Example 1 of U.S. Pat. No. 4,355,154 implies that the catalyst is the last ingredient to be included and all but examples 3, 4, and 7, follow the general procedure in example 1. Examples 4, and 7 follow a procedure resembling the method taught herein, but the monomers are acid anhydrides, in a pre-reaction, conducted in the absence of water. Water in excess of the limits taught by the instant invention is later introduced. Likewise, it is neither taught or claimed in any of these references that solvent can be included to reduce viscosity or prevent crystallization, which was often a problem in the methods of these references, especially at low polymerization temperatures.
The instant invention method is distinguished from that prior art in providing a procedure which provides polymers having significantly higher conversions of the carboxylic acids and polymers having significantly higher molecular weights and which allows the previously undisclosed use of certain solvents.
This invention also provides for low polymerization temperatures, sometimes as low as room temperature which results in suppressing undesired side reactions associated with high temperature polyesterfication such as etherification, oxidation, and the like. This invention provides more latitude on the types of monomers that can be used for the condensation reactions. For example, this invention extends the utility of the emulsion polymerization to the more water soluble carboxylic acids and carbinols commonly used in coating resins without resorting to acid anhydrides which are available for only a limited number of monomers.